I. Field of the Prior Art
This invention relates to a process for making an aluminum alloy sheet suitable for use as can body stock. It also relates to an alloy sheet product suitable for making can bodies.
II. Description of the Prior Art
Aluminum beverage cans are currently made from sheet--from alloys such as alloys designated as AA3004, AA3104 and similar alloys containing Mg, Mn, Cu, Fe and Si as the principal alloying elements. The sheet is generally made by direct chill (DC) casting an ingot (typically 500 to 750 mm thick) of the desired composition, homogenizing the ingot at temperatures of 580 to 610.degree. C. for periods of 2 to 12 hours, and hot rolling the ingot (employing a mill entry temperature of about 550.degree. C.), thereby reducing it to re-roll sheet of about 2 to 3.5 mm thick. The re-roll sheet is then cold rolled in one or more steps to the final gauge (0.26 to 0.40 mm). Various annealing steps may be used in conjunction with the cold rolling, but are frequently not required.
The alloy and processing conditions are selected to give sufficiently high strength and low earing to enable fabrication of a can body by drawing and ironing (D&I) operations, and sufficiently high strength retention after paint baking that the finished can is adequately strong. The strength is believed to be related to the alloying elements and the amount of cold work done to the final sheet, and the amount of earing is believed to be related to the amount of cube texture developed and retained during processing. It is commonly found that homogenization of a DC cast ingot followed by hot rolling is adequate to generate the necessary cube texture, and subsequent cold rolling is used to control the strength.
The use of continuous casting to produce alloy slab (typically 30 mm in maximum thickness) followed by hot rolling the slab directly (essential in a continuous process without homogenization) to make re-roll sheet has decided advantages in the production of sheet products, in that hot rolling can be carried out without having to reheat a large DC cast ingot.
One such process is disclosed, for example, in U.S. Pat. No. 4,614,224. Following the production of re-roll sheet, this process uses a two step cold rolling with an interanneal step.
Another process is disclosed in U.S. Pat. No. 4,976,790 where a two step cold rolling process is disclosed with an anneal before the first cold rolling step and also an interanneal.
In order to achieve useful combinations of earing and strength in the final product, such processes which include intermediate heat treatment steps have generally been required, making them more difficult to use and more expensive for commercial production. Even when such steps are introduced, the final product does not meet both strength and earing requirements of modern can body stock.
U.S. Pat. No. 5,470,405 discloses a process whereby a continuously cast strip is hot rolled and immediately subject to an annealing step (intermediate cooling is not permitted), followed by a rapid quench The resulting product can then be cold rolled to final gauge without interanneal or similar heat treatments. However, the requirement for immediate anneal step and a rapid quench step requires special mill design, and reduces the flexibility of processing.
There is a need therefore for a process for manufacturing can body stock based on a continuous casting process which is capable of producing a strip having properties meeting modern can and can fabrication requirements, which is made cost effective through the elimination of certain process steps such as interanneals and artificial aging previously considered essential, but which retains the flexibility of such processes.